Restore TestMailer

The testmail function on the admin controller is broken because the view for the TestMailer was deleted.  Restored the functionality and made some compatibility changes.  Specifically:

1. Re-added test_mailer view so that TestMailer functions. 
2. Changed TestMailer to take an id, so it can support Resque/Sidekiq.
3. Updated controller to pass an id rather than the user object itself.  
4. Updated controller spec to use persisted user.
5. Added a spec for the TestMailer itself, to prevent regression.